UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Help needed with

frsimen · 12th Feb 2023, 12:20 pm

Hello Mark,

Is the voltage on TP19 0V? If it is, there should be pulses at TP17, so Q6 could be faulty and it would be worth changing it. The switching speeds aren't that high in that area, so you could substitute another NPN small signal transistor if you have one. A BC107/8/9 BC547/8/9 or 2N3904 will all work in that position.

If TP9 is +12V, I would leave Q6 for the time being, as there should be no pulses on TP17 in that condition.

I've spent a while trying to understand how the circuit around ICs 5 and 9 is supposed to work. I've concluded that it's probably a victim of a fault elsewhere, rather than the cause of the fault.

Its purpose is to control the switching of the divide by 160/161. It counts the incoming clock from TP17 and when a particular value is reached it causes IC8a's output to go HIGH. That switches off Q7 and stops the clock pulses at TP17. It also switches the divide by 160/161 to divide by 160. The output from this circuit ends up at TP18 and has been shown to be working.

What should happen next is the problem. The other three ICs, IC21, IC14 and IC25 should still be counting. At some point in the count, they should trigger the J-K flip flop IC10, which will put a 12V pulse on its Q output and, via Q8 onto TP16. This pulse is applies to the preset enable pin on all the counter ICs. The count on ICs 5 and 9 will change and Q7 will be switched on again, returning the clock pulses. The divider will switch to divide by 161.

My earlier measurement of at TP16 looks suspicious, I will have to repeat it later as I think it should be a 10V or more pulse, not what I reported previously.

Even with IC5 and IC9 not working, the rest of the counter should be working, with an error in the divide ratio which will make the final locked frequency off somewhat from what it should be. This means we should turn our attention to IC21, IC14 and IC25 and their associated circuitry (IC6 IC8, IC15, IC22 and IC24).

It's probably best to start with the basics. Check that the 12V supply is present as follows:
IC6, IC8, IC15, IC22, IC24 Pin 14
IC5, IC9, IC21, IC14, IC25 Pin 16 - It will be useful to check IC5 and IC9 for completeness.

Next check that the 10V p-p clock waveform is reaching pins 15 of IC21, IC14 and IC25.

Check that pin 1 of IC5, IC9, IC21, IC14 and IC25 are at 0V, there may be a brief pulse if the circuit is working.

Check for a 10V p-p signal at pins 6, 14 and 2 of IC21. (the output from pin 11 isn't used).

Check for a brief 10V pulse at pin 7 of IC21.

If all pins are good, move on to IC14 and check for a 10V p-p signal at pins 6,11,14 and 2, plus the brief 10V pulse at pin 7 of IC14.

If all is good, move on to IC25. looking again at pins 6,11,14 and 2. The output from pin 7 isn't used.

These tests should narrow down the search area a little more. Report back with anything that isn't as described above.

Paula

12th Feb 2023, 12:20 pm	#87
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Help needed with - Racal Dana 9081 Hello Mark, Is the voltage on TP19 0V? If it is, there should be pulses at TP17, so Q6 could be faulty and it would be worth changing it. The switching speeds aren't that high in that area, so you could substitute another NPN small signal transistor if you have one. A BC107/8/9 BC547/8/9 or 2N3904 will all work in that position. If TP9 is +12V, I would leave Q6 for the time being, as there should be no pulses on TP17 in that condition. I've spent a while trying to understand how the circuit around ICs 5 and 9 is supposed to work. I've concluded that it's probably a victim of a fault elsewhere, rather than the cause of the fault. Its purpose is to control the switching of the divide by 160/161. It counts the incoming clock from TP17 and when a particular value is reached it causes IC8a's output to go HIGH. That switches off Q7 and stops the clock pulses at TP17. It also switches the divide by 160/161 to divide by 160. The output from this circuit ends up at TP18 and has been shown to be working. What should happen next is the problem. The other three ICs, IC21, IC14 and IC25 should still be counting. At some point in the count, they should trigger the J-K flip flop IC10, which will put a 12V pulse on its Q output and, via Q8 onto TP16. This pulse is applies to the preset enable pin on all the counter ICs. The count on ICs 5 and 9 will change and Q7 will be switched on again, returning the clock pulses. The divider will switch to divide by 161. My earlier measurement of at TP16 looks suspicious, I will have to repeat it later as I think it should be a 10V or more pulse, not what I reported previously. Even with IC5 and IC9 not working, the rest of the counter should be working, with an error in the divide ratio which will make the final locked frequency off somewhat from what it should be. This means we should turn our attention to IC21, IC14 and IC25 and their associated circuitry (IC6 IC8, IC15, IC22 and IC24). It's probably best to start with the basics. Check that the 12V supply is present as follows: IC6, IC8, IC15, IC22, IC24 Pin 14 IC5, IC9, IC21, IC14, IC25 Pin 16 - It will be useful to check IC5 and IC9 for completeness. Next check that the 10V p-p clock waveform is reaching pins 15 of IC21, IC14 and IC25. Check that pin 1 of IC5, IC9, IC21, IC14 and IC25 are at 0V, there may be a brief pulse if the circuit is working. Check for a 10V p-p signal at pins 6, 14 and 2 of IC21. (the output from pin 11 isn't used). Check for a brief 10V pulse at pin 7 of IC21. If all pins are good, move on to IC14 and check for a 10V p-p signal at pins 6,11,14 and 2, plus the brief 10V pulse at pin 7 of IC14. If all is good, move on to IC25. looking again at pins 6,11,14 and 2. The output from pin 7 isn't used. These tests should narrow down the search area a little more. Report back with anything that isn't as described above. Paula